This invention relates to a process for preparing polymers whereby foam formation during the removal of residual monomers is inhibited.
There are three common, general methods of polymerization. When a single monomer or a homogeneous mixture of two or more monomers is polymerized without a solvent or diluent, and as a single liquid phase, to form a solid polymeric block, the process is called "mass" or "homogeneous" polymerization. When one or more water-immiscible monomers and water-soluble catalyst or water-soluble initiators are dispersed in water by means of a "surface active" or emulsifying agent to form a latex-like dispersion of fine particles of the polymeric body, the process is called "emulsion" polymerization. The third process comprises mixing the monomers and water-immiscible and monomer soluble catalyst or initiators with water using a dispersing or suspending agent, and maintaining the dispersion or suspension by continued agitation at a temperature known to induce polymerization. The present invention relates to this third process, which is called "suspension" polymerization.
In the suspension process, starting monomers are suspended in an aqueous phase comprising a suspending agent and a polymerization initiator. Polymerization of the starting monomers is initiated and generally carried out in the presence of heat and agitation. Polymerization is allowed to continue until the desired degree of conversion of starting monomer to polymer is achieved. The resulting product, in the form of an aqueous suspension or dispersion, invariably contains a significant quantity of residual monomers and other volatile, non-monomeric impurities, such as organic solvents and the like. In many instances, it is desirable to remove substantially all of such residual monomers and impurities in the dispersion.
Conventionally, residual monomers have been removed by simply venting the reaction vessel, or by heating and/or reducing the pressure within the reaction vessel. However, during such removal of the residual monomers, severe foam formation occurs in the dispersion which makes the residual monomer removal a very slow process. Foaming also causes carryover of polymer solids into the vacuum system, causing plugging of process lines.
It would be desirable to provide a process for preparing a polymer whereby the formation of foam during the removal of residual monomers is inhibited.